1. Field of the Invention
The present invention relates to an electroluminescent device of compound semiconductor. More particularly, it is concerned with an electroluminescent device which comprises an electroluminescent part of a compound semiconductor disposed on a compound semiconductor substrate.
2. Description of the Prior Art
Group II-VI compound semiconductors, such as zinc sulfide (ZnS) and zinc selenide (ZnSe), are used for making short-wavelength electroluminescent devices which produce light having a blue color (IEEE TRANSACTIONS ON ELECTRON DEVICES, Vol. ED-30, p.271 (1983)). FIG. 6 illustrates the basic construction of the conventional electroluminescent devices manufactured from those materials. FIG. 6 is a cross sectional view of a metal-insulator-semiconductor (MIS) type electroluminescent device using ZnS, in which elements 100 and 107 are lead wires (e.g. Au wires), element 101 is a metal electrode (e.g. In or Al), layer 102 is a low-resistance n-type ZnS substrate, layer 103 is an epitaxial low-resistance n-type ZnS conductive layer, layer 104 is an epitaxial low-resistance n-type ZnS light-emitting layer, layer 105 is an epitaxial high-resistance ZnS layer for carrier injection, and element 106 is a metal electrode (e.g. Au). The layers 103, 104 and 105 form an electroluminescent part to which a voltage is applied through the ZnS substrate 102. Therefore, it is impossible to use a bulk single crystal of high-resistance ZnS as the substrate ZnS 102 which is, for example, gown by an iodine transport method. Thus, a bulk single crystal is usually used as the substrate which has been subjected to a long period of heat treatment in a solution of 90% Zn and 10% Al for about 100 hours at a high temperature of about 1000.degree. C. to lower its resistivity to a level of 10 to 1 .OMEGA..multidot.cm and cut to have a thickness of 300 to 1000 microns.
The epitaxial ZnS layers (electroluminescent part) consisting of three layers on the substrate is formed by, for example, molecular-beam epitaxy (MBE). The epitaxial conductive layer 103 and the epitaxial light-emitting layer 104 are obtained by using Al or Cl, etc., as an impurity having a resistivity of 10.sup.-2 to 10.sup.-3 .OMEGA..multidot.cm. The layers are composed of a high quality crystal having a resistivity which is lower by as many as two to four figures of magnitude than the resistivity of the substrate 102.
It has also been necessary to carry out heat treatment at 450.degree. C. in a highly pure gas atmosphere for a period of several seconds to several minutes after the formation of the metal electrodes 101 and 107 when forming ohmic contacts with the n-type ZnS substrate 102.
In summary, the conventional device manufacturing process has the drawback of requiring a long period of heat treatment at a high temperature when preparing the n-type ZnS substrate of low resistance. Also, heat treatment of the electrodes on the substrate at a temperature which is approximately equal to, or even higher than, the temperature at which the epitaxial layers, including the light-emitting layer, are grown, as hereinabove pointed out.
Another drawback of the conventional art has been that the majority of loss in the semiconductor portions of the electroluminescent device occurs in the substrate, because the substrate has a resistance which is as high as 1 to 10.OMEGA., while the light-emitting and conductive layers have a total resistance of 10.sup.-3 to 10.sup.-4 .OMEGA.. Further, the use of a ZnS substrate which is not satisfactorily low in resistivity has brought about a serious drawback from the standpoint of practical applications. When finely divided patterns are formed on the substrate, or very small chips having a size in the order of 100 microns are formed, in order to make a monolithic display device, the luminescent portions or chips have a high serial resistance.
These drawbacks have also been found to occur in the electroluminescent devices made by employing a substrate formed from a Group III-V compound semiconductor, such as GaAs (IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-24, No. 7, July 1977).
The present invention has been made to overcome the drawbacks which have hereinabove been pointed out.